Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system may allow the vapors to be purged into an engine intake manifold for use as fuel. The purging of fuel vapors from the fuel vapor canister may involve opening a canister purge valve coupled to a conduit between the fuel vapor canister and the intake manifold.
In an example approach shown by Bugin et al. in U.S. Pat. No. 5,005,550, a fuel system canister is purged using either vacuum from an engine intake manifold or vacuum generated by an aspirator. The aspirator is coupled in a compressor bypass passage and vacuum is generated by the aspirator (also termed ejector herein) via motive flow through the compressor bypass passage. During a purge operation into the intake manifold, vacuum or negative pressure in the intake manifold may be applied to the fuel system canister via a solenoid valve. Alternatively, when manifold vacuum is insufficient, such as during boosted conditions, aspirator vacuum may draw desorbed fuel vapors along with purge air from the canister into the compressor bypass passage.
The inventors herein have identified potential issues with the above approach. As an example, when the fuel vapor canister is saturated, purge flow from the fuel vapor canister into the aspirator may contain more fuel vapors than desired. For example, during transient conditions such as rapid deceleration, fuel demand from the engine may be significantly lower. However, aspirator vacuum may continue to extract fuel vapors from the fuel vapor canister and may supply these fuel vapors to the engine. Excess fuel than desired can increase combustion instability, and lead to adverse effects such as loss of engine power and efficiency. These issues may be of higher concern when motive flow through the aspirator is not regulated by a valve.
The inventors herein have recognized the above issues and identified an approach to at least partly address the issues. In one example approach, a method for a boosted engine comprises, during boosted conditions, flowing stored fuel vapors from a canister into an ejector coupled in a compressor bypass passage, the flowing bypassing a canister purge valve, and responsive to a canister load higher than a threshold, closing a canister vent valve coupled to the canister, and discontinuing flowing stored fuel vapors from the canister into the ejector. Thus, saturated fuel vapors may not be delivered to the ejector by closing the canister vent valve.
For example, a boosted engine may include an ejector positioned in a compressor bypass passage. A suction port of the ejector may be fluidically coupled to a fuel vapor canister. The fuel vapor canister may be fluidically coupled to atmosphere via a canister vent valve. The fuel vapor canister may also be fluidically coupled to an intake manifold of the boosted engine via a canister purge valve. The fuel vapor canister may communicate with each of the canister purge valve, the atmosphere, and the suction port of the ejector via distinct and separate passages. The ejector may generate vacuum due to the flow of compressed air in the compressor bypass passage. As such, the ejector may be an un-valved ejector such that motive flow of compressed air through the ejector may not be regulated actively. Herein, the un-valved ejector may not include a valve controlled by an engine controller situated at any of the suction port of the ejector, motive inlet of the ejector, or motive outlet of the ejector. Ejector vacuum may draw stored vapors from the fuel vapor canister into an inlet of the compressor. Thus, the fuel vapor canister may be purged during boosted engine conditions. However, when a load of the fuel vapor canister is determined to be higher than a threshold, purging of the fuel vapor canister via the ejector may be discontinued by closing the canister vent valve. Specifically, the canister vent valve may be adjusted to a fully closed position to cease canister purge. Further, the canister vent valve may be closed in response to the load of the fuel vapor canister being higher than the threshold during lower engine air flow conditions such as deceleration.
In this way, vapor purge from a fuel vapor canister into an ejector may be controlled in a simpler manner. The canister vent valve may be used to control purge flow into the ejector in engine systems including un-valved ejectors. By controlling purge flow into the ejector via the canister vent valve, an additional shut-off valve for the ejector may not be utilized, saving costs. Further, since purge flow into the ejector is based on canister load, the engine may not receive rich vapors when fuel demand is lower. Accordingly, engine performance may be enhanced and drivability may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.